The present invention relates to an optical interferometric angular rate meter or fiber optic gyro which measures an input angular rate by detecting that the phase difference between light beams propagating through such a looped optical path as an optical fiber coil in opposite directions varies with the input angular rate. More particularly, the invention pertains to a fiber optic gyro with a self-diagnostic capability whereby the gyro is allowed to make a self-check of a failure and indicate its occurrence to a host system or send thereto a self-checking signal when the gyro performance or function is impaired or degraded.
A description will be given first, with reference to FIG. 1, of a conventional fiber optic gyro. Light I from a light source 11 passes through an optical coupler 12, a polarizer 13 and an optical coupler 14 and then enters into an optical fiber coil 15 from its opposite ends, which coil is used as an optical path. A light source driver 23 drives the light source 11, and it is composed of, for instance, a circuit which merely supplies current to the light source 11 and a circuit which monitors a portion of the quantity of light from the light source 11 by means of a photodetector and automatically controls the light source 11 to emit a fixed quantity of light at all times.
A reference signal generator 24 applies a modulation signal Sp via a modulator driver 22 to a phase modulator 16. The clockwise and counterclockwise light beams which propagate through the optical fiber coil 15 are phase modulated by the phase modulator 16 inserted between one end of the optical fiber coil 15 and the optical coupler 14. The phase-modulated light beams are both combined by the optical coupler 14 with the light beam emitted from the other end of the optical fiber coil 15 and interfere with each other, and the interference light is provided via the polarizer 13 to the optical coupler 12, by which it is branched to a photodetector 17.
Letting the phase modulation by the phase modulation signal Sp be represented by P(t)=Asin.omega..sub.p t, the output Vp of the photodetector 17 is expressed by the following equation: EQU Vp=(I/2)K.sub.op K.sub.pd {1+cos.DELTA..PHI.(.SIGMA..epsilon..sub.n (-1).sup.n J.sub.2n (x)cos2n.omega..sub.p t')-sin.DELTA..PHI.(2.SIGMA.(-1).sup.n J.sub.2n+1 (x)cos(2n+1).omega..sub.p t')} (1)
In the above,
.SIGMA.: a summation operant from n=0 to infinity;
t': t-.tau./2
.epsilon..sub.n : 1 for n=0, .epsilon..sub.n =2 for n.gtoreq.1
K.sub.op : optical loss on the emitted light I from the light source 11 which is caused by the optical path to the photodetector 17;
K.sub.pd : a constant which is determined by a photoelectric conversion coefficient, an amplifier gain, etc.;
I: the quantity of light emitted from the light source 11;
Io: the maximum quantity of light which reaches the photodetector (Io=Kop.multidot.I)
J.sub.n (x): a Bessel function of the first kind, x=2Asin.pi.f.sub.p .tau.;
A: a modulation index;
.DELTA..PHI.: the phase difference between the clockwise and counterclockwise light beams in the optical fiber coil 15;
.omega..sub.p : an angular frequency of the phase modulation (.omega..sub.p =2.pi.f.sub.p, but in the following description, .omega..sub.p may sometimes be called a frequency as an equivalent to the frequency f.sub.p, for the sake of brevity);
.tau.: the time for propagation of light through the optical fiber coil.
In order that the phase difference .DELTA..PHI. corresponding to the applied angular rate may be detected, with high sensitivity, from the output of the photodetector 17 expressed by Eq. (1), it is necessary to detect a sin.DELTA..PHI. component which maximizes the gradient of the phase difference .DELTA..PHI. in the neighborhood of zero. As shown in Eq. (1), sin.DELTA..PHI. is a coefficient of odd harmonic components of the frequency .omega..sub.p, and hence a desired one of these odd harmonic components needs only to be detected. This can be done by a synchronous detection with a reference signal of the same frequency as that of the desired odd harmonic component. In FIG. 1 the output of the photodetector 17 is applied to a synchronous detector 18, wherein it is multiplied by a reference signal Sr of the same frequency as the phase modulation frequency .omega..sub.p to generate frequency components corresponding to the sum of and the difference between the photodetector output and the component of the frequency .omega..sub.p (the term with the coefficient sin.DELTA..PHI. for n=0 ) in Eq. (1). At the same time, the other components (including a DC component as well) in Eq. (1) are all provided as the fundamental and higher harmonic components of the frequency .omega..sub.p. The output of the synchronous detector 18 is applied to a low-pass filter 19 to cut off the detector output except for the difference component (the DC component), and only the DC component is derived at a proper gain, thereafter being provided as the output of the fiber optic gyro (hereinafter referred to as an FOG output) to a terminal 21.
The FOG output Vo is expressed by the following equation: ##EQU1## where K.sub.A1 is a gain.
The phase difference .DELTA..PHI. between the two light beams represents a Sagnac phase difference .DELTA..PHI..sub.s which is caused by the application of an angular rate .OMEGA. to the optical fiber coil 15, and this phase difference is expressed by the following equation: EQU .DELTA..PHI..sub.s =4.pi.RL.multidot..OMEGA./C.lambda. (3)
where C is the velocity of light, .lambda. is the wavelength of light in a vacuum, R is the radius of the optical fiber coil 15 and L is the length of the optical fiber forming the optical fiber coil 15. Hence the input angular rate .OMEGA. can be detected by measuring the output Vo of the low-pass filter 19.
As described above, when the functions and performance of respective parts of the fiber optic gyro are normal, the input angular rate can be detected by measuring the FOG output Vo. In the case of the prior art, however, even if the fiber optic gyro develops an abnormality and operates at reduced performance, produces no output or continues to output an abnormal voltage, it is impossible to judge from the FOG output alone whether the gyro is normal or abnormal--this incurs the possibility that the system using the fiber optic gyro gets into danger.